DECREASED disaccharidase activity in the small intestine may be primary, or secondary to other small intestinal diseases.1,2 The diagnosis is established by appropriate oral carbohydrate tolerance tests and assays of the mucosal disaccharidases of the small intestine. The work reported here concerns: (1) the oral lactose tolerance test in normal humans given varying amounts of the test carbohydrate; (2) our normal range of small intestinal disaccharidase and alkaline phosphatase activities; and (3) additional data on the distribution of these enzymes in the duodenum and jejunum. Material and Methods Patients. —Twenty-five individuals were selected for this study on the bases of a negative history of milk or disaccharide intolerance and/or normal disaccharide tolerance tests. The subjects were then classified into two groups. Group 1 consisted of 19 presumably healthy individuals ranging in age from 21 to 46 years; 14 men and 5 women. Seventeen were Caucasian and two were